Electromagnetic coupling apparatus



Sept. 1, 1970 v E. F. HELDT I ELECTROMAGNETIC COUPLING APPARATUS Filed Sept. 24, 1968 2 Sheets-Sheet 1 l9 ELECTRICAL ..CONDUCT|VE SHEET CIRCUIT FIG. 1

FIG. 3

FIG. 2

Sept. 1, 1970 I E. F. HELDT 3,526,856

ELECTROMAGNET I C COUPLING APPARATU S Filed Sept. 24, 1968 2 Sheets-811eet l ELECTRICAL CIRCUIT 3,526,856 ELECTROMAGNETIC COUPLING APPARATUS Edward F. Heldt, Rockaway Beach, N.Y., assignor to Hazeltine Research, Inc., a corporation of Illinois Filed Sept. 24, 1968, Ser. No. 762,062

Int. Cl. H03h 1/00 U.S. Cl. 33324 11 Claims ABSTRACT OF THE DISCLOSURE An electromagnetic coupling apparatus consisting of a conductive member positioned adjacent to a piece of magnetic insulator material such as an yttrium iron garnet (YIG) sla'b or bar. The conductive sheet has an aperture whose width is less than .03 of the free space wavelength of the energy to be coupled to or from the magnetic insulator material. A thin conductive Wire traverses the apert-ure on the side of the conductive member opposite the magnetic insulator material. The conductive sheet serves to localize the source and the only coupling occurs through the small aperture in the conductive member. Alternative arrangements are also covered.

The present invention relates generally to apparatus for coupling energy to or from magnetic insulator material and particularly to localized couplers for efficiently coupling electromagnetic energy to or from the magnetic insulator.

In recent years substantial time and effort has been spent in analyzing and understanding gyromagnetic wave propagation in magnetic insulator materials, such as ferrites and garnets. Because of its low loss characteristics yttrium iron garnet (YIG) has been of particular interest to those working in this field. However, one of the major problems encountered in making practical usage of wave propagation in YIG as well as in other magnetic insulator materials has been the difiiculty in achieving efiicient, localized coupling of energy into and out of the material. Both acoustic and electromagnetic coupling have been attempted and achieved to some extent but both have had drawbacks. Acoustic couplers generally have been complicated and costly to fabricate and obviously only launch acoustic waves. Electromagnetic couplers are easier and cheaper to fabricate but significant shielding problems have limited their usage.

Objects of the present invention therefore are to provide new and improved simple localized coupling structures for coupling electromagnetic energy to or from magnetic insulator material.

In accordance with the present invention there is provided a coupling structure for coupling electromagnetic energy having a predetermined range of frequencies to or from magnetic insulator material which comprises a piece of magnetic insulator material and a conductive member contiguous with one face of the magnetic insulator material and which has an aperture whose width is substantially less than the free space wavelength of the predetermined frequencies. The invention further comprises a thin conductive wire a portion of which traverses the aperture transverse to its width in close proximity to the magnetic insulator material and which has one end connected to the conductive member for causing substantially all the return currents associated with the current flow in the wire to be shielded from the magnetic insulator material by the conductive member, whereby efficient localized coupling can be provided between the wire and the magnetic insulator material through the aperture.

For a better understanding of the present invention together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings:

FIG. 1 illustrates an electromagnetic coupling structure constructed in accordance with the present invention;

FIG. 2 is an exploded view of a section of the FIG. 1 coupling structure;

FIG. 3 is an exploded view of an alternative section corresponding to the section shown in FIG. 2; and

FIGS. 4a and 4b illustrate a second electromagnetic coupling structure constructed in accordance with the present invention.

DESCRIPTION OF FIGS. 1-3

FIGS. 1-3 illustrate an electromagnetic coupling structure for coupling energy, such as electromagnetic energy, having a predetermined range of frequencies to or from the material 10 constructed in accordance with the present invention. The apparatus consists of the stated piece of material 10 such as magnetic insulator material. As previously stated the most commonly used magnetic insulator is yttrium iron garnet (YIG). Generally, the material is either in the form of a slab in which the width W is substantially greater than the height H, or a bar in which the height and the width are in the same order of magnitude, or a cylindrical rod. FIG. 1 illustrates the slab configuration.

Analysis of the types and reasons for wave propagation in YIG material has been the subject of a vast amount of investigation and there have been many articles written about the various types of waves propagated. Although propagation in YIG is still not thoroughly understood and is under continuing investigation, it is generally agree that electromagnetic waves propagate in the material. It is the coupling of such electromagnetic energy to and from the YIG material to which the present invention is directed.

The apparatus further comprises a conductive member 11 illustrated as a thin sheet of conductive material, such as copper foil, contiguous with the piece of magnetic insulator material 10. For optimum coupling the sheet of conductive material should be placed as close to the YIG slab 10 as practical, preferably in direct contact with the YIG. In this example, the conductive member 11 is shown as having a perpendicular base portion for purposes of mechanical support. An alternative approach is to deposit the copper foil on the face of the magnetic insulator 10 by thick or thin film techniques.

The conductive member 11 has an aperture 12 whose width is substantially less than the free space wavelength of any of said predetermined frequencies. The shape and relative size of the aperture are more clearly illustrated in FIG. 2 which is an expanded view of the section 13 and in FIG. 3 which is an alternative construction corresponding to FIG. 2. FIG. 2 illustrates a conductive member 11 with a circular aperture 12 having a width 14. FIG. 3 illustrates a conductive member 11' with an aperture 12" which is in the form of an elongated slot having a width 14' and a length 15';

The apparatus also includes a transmission line means illustrated as a portion of a strip-line 16 having a grounded conductor 17 and an ungrounded conductor 18 separated by dielectric material. At one end the transmission line is connected to an electrical circuit 19 by way of the strip-line connector 20 and coaxial cable 25. At the other end the grounded conductor 17 is connected to the sheet of conductive material 11 and the ungrounded conductor 18 is connected to a thin conductive wire 21 via the hole 22 in the grounded conductor 17. The conductive wire 21 is positioned on the side of the copper foil 11 opposite the magnetic insulator so that wire 21 is electromagnetically shielded from the magnetic insulator 10 by the conductive member 11, except for the portion of the wire 21' which traverses the aperture 12. As more clearly illustrated in FIGS. 2 and 3, the wire 21 traverses the aperture 12 transverse to the width of the aperture and has one end connected to the conductive member 11 at junction 23. Both the wire 21 and the grounded conductor 17 are connected to the side of the conductive member 11 opposite the magnetic insulator 10, thereby causing substantially all the return or ground currents associated with current flow in the wire 21. to flow on the same side of the copper foil 11 as the wire 21.

Conductive Wire 21 may be a length of conventional circular cross-sectional wire or a thin flat piece of conductive foil. In addition, the portion 21 or 21" of wire 21 may be deposited directly on the end face of magnetic insulator 10 by vacuum deposition with an insulating film such as silicon monoxide isolating the two conductors.

The apparatus further includes means 24 for electrically insulating the conductive wire 21 from the conductive member 11 in the region of the aperture 12. Means 24 may be a thin film of dielectric material such as Teflon. The sheet must be extremely thin in order to insure optimum coupling between the magnetic insulator 10 and the wire 21. The purpose of the dielectric sheet is to prevent the conductive member from coming in electrical contact with the conductive member on both sides of the aperture in order to avoid shorting the section 21' or 21 of the wire across the aperture 12.

OPERATION OF FIGS. 1-3

The coupler of FIG. 1 is reciprocal in nature; that is to say, the same apparatus may be an input device coupling electromagnetic energy from the wire 21 to the magnetic insulator 10 or an output device coupling energy from the magnetic insulator 10 to the wire 21. The only difference is that as an input device the electrical circuit 19 includes a signal generator and as an output device the electrical circuit includes a utilization load. For ease of understanding, the following discussion describes the coupler as an input device, but it will be obvious to those skilled in the art that the same apparatus can be used as an output device with reciprocal energy propagation and current flow occurring.

The two basic requirements of an electromagnetic coupler are somewhat conflicting. On the one hand, the de vice must efliciently couple energy between the wire radiator 21 and the magnetic insulator 10. The other requirement is that the energy coupled to the magnetic insulator 10 be only the intended electromagnetic energy; i.e. that the source be localized. An unshielded wire provides very eflicient coupling. However, the additional coupling of spurious electromagnetic energy degrades the quality of such a coupler to the point where the location and nature of the source is not clearly defined.

It has been discovered that it is possible to place an extremely small aperture in a thin sheet of conductive material such as copper and achieve eflicient localized coupling through the aperture. A conductive sheet covering a large portion of the face of the YIG would be expected to short out the relevant field. The reason why eflicient localized coupling does occur through the small aperture is not yet fully understood.

The source is localized primarily due to the fact that the conductive member 11 confines the magnetic fields produced by the wire radiator 21 and substantially all the return current flow is limited to the wire side of the conductive member. If there were a substantial amount of concentrated current flow on the magnetic insulator side of the conductive member 11, corresponding electromagnetic waves would be produced which would be a source of spurious coupling. By connecting both the Wire radiator 21 and the grounded conductor 17 of the transmission line to the same side of the conductive member 11, most of the return or ground current associated with current flow in the wire radiator flows 0n=the wire side of the conductive member. Minimal current flow will occur on the magnetic insulator side of the conductive member. Therefore, the only substantial coupling to the YIG material results from the magnetic field produced by the portion of the wire 21' of the wire radiator 21 which traverses the aperture 12, thereby providing a very localized source.

In order to couple electromagnetic energy into the magnetic insulator 10, high frequency current is caused to flow from the electrical circuit 19 through the coaxial cable 25, the strip-line connector 20, strip-line 16, and thin wire conductor 21. Depending on the usage to which the magnetic insulator is to be placed, the current supplied by source 19 consists of either a single frequency or predetermined frequency spectrum. In either case, the present practical range of signal frequencies is considered to be from about 350 mI-Iz. to 10 gHz. At present YIG material is generally used with signal frequencies between 1 gI-lz. and 2.5 gHz.

Current flow in the thin wire conductor 21 produces a corresponding electromagnetic field in the vicinity of the wire. In the absence of the conductive member 11 the 21 and the conductive member .11 does not appreciably.

degrade the coupling between the magnetic insulator 10 and the current flowing in the wire 21. For optimum coupling, the wire 21 and the magnetic insulator 10 are positioned as close as possible to the opposite sides of the aperture 12. In one coupler which was successfully constructed and tested, the wire 21 was butted against a sheet of Teflon, .001 inch thick, which was affixed to a sheet of copper foil .002 inch thick. A YIG slab was placed in physical contact with the copper foil on the.

opposite side of the aperture. The Teflon sheet 24 and wire radiator were forced into the aperture so that the wire radiator was separated from the YIG material only by the thickness of the Teflon sheet, .001 inch. It is expected that even more eflicient coupling could be achieved by notching the Teflon sheet in the region of the aperture so that the wire 21 is placed in direct contact with the magnetic insulator 10 while still preventing the wire from shorting across the aperture 12.

Current caused to flow through the wire 21 flows through the conductive member 11 to which the Wire 21:

is directly connected at junction 23. This return current follows the path of least resistance through conductive member 11 to the ground plane 17. Having the ground plane 17 connected to the wire side of the conductive member 11 causes substantially all the return current to flow on the wire side of the conductive member. Substantially all electromagnetic fields resulting from; the

return currents are therefore shielded from the magnetic insulator 10 by conductive member 11 and accordingly no significant spurious coupling of electromagnetic energy. results from return current flow. It is immaterial that the ground plane 17 is also connected to the magnetic insulator side of the conductive member 11 since, as stated, the current follows the path of least resistance.

The following is a table of the relevant parameters of two couplers which were successfully constructed and tested, one having a circular aperture as illustrated in FIG. 2 and the other having an elongated slot aperture as illustrated in FIG. 3.

Both of these couplers provided excellent results. However, they are merely intended as representative examples of the present invention and the dimensions given are not critical. For a frequency range of 350 mHz. to gHz. the width 14 or 14' is less than one-eighth of an inch. The value chosen involves a compromise between the above mentioned conflicting requirements. The larger aperture provides better coupling while the smaller aperture provides better isolation. The width of the wire can be from one to ten mils, the finer wire being used for the smaller aperture. Generally speaking, the ratio of the width of the aperture to the width of the wire is in the order of 10:1.

The ratio of the shortest free space wavelength of the signal frequencies utilized in conjunction with the above mentioned couplers and the width of the aperture is almost 200: 1. That is to say that the width of the aperture is less than .006 times the free space wavelength. Although the compromise between efficient coupling and necessary isolation must be considered, it is preferred that for a particular coupler the width of the aperture be less than .03 times the free space wavelength.

It has been discovered that the amount of coupling for a given aperture is only slightly frequency sensitive. The same aperture can be utilized to efiiciently couple a relatively large portion of the frequency spectrum. Even when the signal frequencies to be utilized are specified, it is impossible to specify a preferred width for the aperture because the above mentioned compromise must be considered. On the other hand once the frequency spectrum, the maximum loss and the amount of shielding are known, an optimum width aperture can be empirically determined.

The slot arrangement illustrated in FIG. 3 provides somewhat better coupling than a comparable width circular aperture with no appreciable degradation in localization. Since the width of the slot is the same as the width of the hole, the field in the horizontal direction is confined substantially the same amount. Therefore, in the horizontal direction the source is localized substantially to the same degree in both the circular aperture and slot configurations. On the other hand, in the slot configuration the source approximates a line source which provides superior coupling to the circular hole configuration which approximates a point source. Of the two above mentioned couplers successfully constructed and tested, the slot configuration had less than 5 db additional loss as compared to an unshielded wire radiator, while the circular aperture had less than 10 db additional loss. These losses must be evaluated in light of the fact that the insertion loss of YIG may be db or better.

DESCRIPTION OF FIGS. 4a AND 4b FIGS. 4a and 4b illustrate another embodiment of an electromagnetic coupling structure for coupling electromagnetic energy having a predetermined range of frequencies to or from magnetic insulator material 10 constructed in accordance with the present invention. The structure illustrated in FIG. 4a includes the stated piece of magnetic insulator material 10, such as a piece of single crystal YIG, a block of conductive material 27, and a section of strip-line 30, shown coupled via connector to electrical circuit 36. FIG. 4a also illustrates a plastic channel 26 secured to the bottom of the block of conductive material 27 for holding the piece of YIG -10 in place against the block of conductive material 27.

FIG. 4b is a blown-up view of the FIG. 4a structure in which the YIG material 10 and the plastic holder 26 have been removed in order to more clearly illustrate the coupling structure. As is shown in FIG. 4b, the face 27a of the block of conductive material which is in contact with the YIG material 10 has a narrow groove 28 along the full height L of the face 27a. Groove 28 may be machined out by any conventional technique. The groove is substantially filled with dielectric material 29 such as Teflon.

The apparatus further comprises transmission line means 30 having a grounded conductor 31 connected to the block of conductive material 27 and an ungrounded conductor 32 coupled to a thin conductive wire 33. The thin conductive wire 33 is positioned along the length L of the groove 28. The conductive wire 33 is isolated from the block of conductive material along the length of the groove 28 by the dielectric material 29 which is positioned between the wire 33 and the block of conductive material 27. The other end of the wire 33 is afiixed to the block of conductive material at junction 34 for causing substan tially all the return currents associated with current flow in the wire conductor 33 to be shielded from the YIG material 10 by the block of dielectric material 27. For optimum coupling, it is desirable to have the YIG material 10 in direct contact with the wire conductor 33. Therefore, it is preferable to have the wire 33 run along the front face 27a of the block of conductive material 27. This may be achieved by filling the groove 28 entirely with dielectric material 29 and then removing a small portion of the dielectric material 29 sufficient to accommodate the conductive wire 33 so that the wire 33 is fiush with the front face 27a of the conductor 27 and in direct contact With the YIG material 10.

OPERATION OF FIG. 4

The coupler of FIG. 4 operates in substantially the same manner as the previously described FIG. 1 coupler. Operating as an input device, high frequency current is coupled to the connector 35 from an external source 36 through the ungrounded conductor 32 of strip-line 30 to the thin conductor wire 33. The electromagnetic field produced by current flow in the wire couples to the YIG material 10.

The wire 33 is grounded at the junction point 34 so that substantially all the return current associated with current flow in the wire flows in the side walls of the groove 28 to the ground plane 31. Some of the current may flow on the front face 27a but it is widely dispersed so that it will not cause any significant coupling to the YIG material 10.

The parameters specified in conjunction with descriptions of the FIG. 1 embodiment are applicable to the FIG. 4 configuration. The wire size generally will be in the range of one to ten mils and the width of the groove will be less than one-eighth of an inch. Generally, the ratio of the 1width of the groove to the wire size is in the order of 10:

A coupler as illustrated in FIG. 4 was successfully constructed and tested. The groove was filled with Teflon material and the wire was positioned within the Teflon so as to be in direct contact with the YIG material 10. The wire was three mil wire and the width of the groove W was 30 mils. The signal frequency was from 1.1 gHz. to 2.4 gHz. The conductive member 27 was a small brass block. This coupler produced very efficie nt localized coupling. The insertion loss was even less than either of the FIG. 1 couplers with no appreciable change in localization. It is expected that even better results can be achieved by using a more conductive material in place of the brass block or by plating the brass with a layer of good conductive material such as silver.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. .A COupling structure for coupling electromagnetic energy having a predetermined range of frequencies to or from magnetic insulator material, comprising:

a piece of magnetic insulator material;

a conductive member contiguous with one face of said magnetic insulator material covering a substantial portion thereof, and having an aperture whose width is substantially less than the free space wavelength of said predetermined frequencies and whose crosssectional area is substantially less than the surface area of said face of the magnetic insulator material;

and a thin conductive wire, a portion of which traverses the aperture transverse to its width in close proximity to said magnetic insulator material and which has one end connected to said conductive member for causing substantially all the return currents associated with the current flow in said wire to be shielded from the magnetic insulator material by the conductive member;

whereby eflicient localized coupling can be provided between said wire and said magnetic insulator material through said aperture.

2. A coupling structure as specified in claim 1 in which the width of the wire is substantially less than the width of the aperture in said conductive member.

3. A coupling structure as specified in claim 2 which additionally includes means for insulating the conductive wire from the conductive member in the region of said aperture.

4. A coupling structure as specified in claim 2 which additionally includes a thin sheet of dielectric material positioned between the conductive member and the conductive wire in the region of said aperture.

5. A coupling structure for coupling electromagnetic energy having a predetermined range of frequencies to or from yttrium iron garnet, comprising:

a piece of yttrium garnet (YIG);

a thin sheet of conductive material positioned adjacent and parallel to one face of the YIG material covering a substantial portion thereof, and having an aperture whose width is less than .03 times the free space wavelength of said predetermined frequency and whose cross-sectional area is substantially less than the surface area of said face of the YIG material;

a transmission line means having a grounded conductor connected to the sheet of conductive material and an ungrounded conductor;

a thin conductive wire shielded from the magnetic material by the sheet of conductive material except for a portion of the wire which traverses the aperture transverse to its width and having a Width which is substantially less than the width of said aperture, said wire coupled at one end to the ungrounded conductor of said transmission line means and at the opposite end to the same side of the sheet of conductive material as the grounded conductor of said transmission line for causing substantially all the return currents associated with current flow in said thin wire to flow on that side. of said sheet of conductive material;

and a thin layer of dielectric material separating the thin conductive wire from the sheet of conductive material;

whereby eflicient localized coupling can be provided between said wire and the YIG material through said aperture.

6. A coupling structure as specified in claim 5 in which the width of the conductive wire is betweenuOOl inches and .010 inch, the width of the aperture is less than oneeighth of an inch and the ratio of the Width of the aperture to the width of the wire is in the order of 10:1.

7. A coupling structure as specified in claim 6 in which the thickness of the sheet of dielectric material is in-the order of .001 inch and the thickness of the dielectric material is substantially the entire separation between the wire and the magnetic insulator material.

8. A coupling structure for coupling electromagnetic energy having a predetermined range of frequencies to or from yttrium iron garnet, comprising:

a piece of yttrium iron garnet (YIG);

a block of conductive material having a substantially fiat face contiguous with one face of the YIG material and having a narrow groove along one dimension of said flat face;

transmission line means having a grounded conductor connected to the block of conductive material, and an ungrounded conductor;

and a thin conductive wire positioned along the length of the groove in the block of conductive material, isolated from the block of conductive material along the length of the groove and coupled at one end of the groove to the ungrounded conductor of said transmission line and coupled at the opposite end of the groove to the block of conductive material for causing substantially all the return currents associated with current flow in said thin wire to be shielded from the YIG material by the block of conductive material;

whereby efiicient localized coupling can be provided between the wire and the YIG material through the groove in the conductive material.

9. A coupling structure as specified in claim 8 in which the conductive wire is isolated from the block of conductive material along the length of the groove by dielectric material positioned in the groove between the conductive wire and the block of conductive material.

10. A coupling structure as specified in claim 9 in which the width of the conductive wire is between .001 inches and .010 inch, the width of the groove is less than one-eighth of an inch, the ratio of the width of the groove to the width of the Wire is in the order of 10:1 i

and the height of the groove is substantially equal to the height of the YIG material.

11. A coupling structure as specified in claim 10 in which the YIG material is in physical contact, with the wire conductor.

References Cited UNITED STATES PATENTS HERMAN KARL SAALBACH, Primary Examiner P. L. GENSLER, Assistant Examiner US. Cl. X.R.

2/1967 Denton et al. 33330 4/1966 Schloemann 330-5 X 

